1. Field of Invention
The aspects of the present disclosure relate generally to the field of electric lighting, and in particular to ballast circuits used to drive gas-discharge lamps.
2. Description of Related Art
A gas-discharge lamp belongs to a family of electric lighting or light generating devices that generate light by passing an electric current through a gas or vapor within the lamp. Atoms in the vapor absorb energy from the electric current and release the absorbed energy as light. One of the more widely used types of gas-discharge lamps is the fluorescent lamp which is commonly used in office buildings and homes. Fluorescent lamps contain mercury vapor whose atoms emit light in the non-visible low wavelength ultraviolet region. The ultraviolet radiation is absorbed by a phosphor disposed on the interior of the lamp tube causing the phosphor to fluoresce, thereby producing visible light.
Fluorescent lamps exhibit a phenomenon known as negative resistance, which is a condition where increased current flow decreases the electrical resistance of the lamp. If a simple voltage source is used to drive a fluorescent lamp, this negative resistance characteristic leads to an unstable condition in which the lamp current rapidly increases to a level that will destroy the lamp. Thus, a fluorescent lamp needs to be driven from a power source that can control the lamp current. While it is possible to use direct current (DC) to drive a fluorescent lamp, in practice, alternating current (AC) is typically used because it affords easier and more efficient control of the lamp current. The current controlling circuits used to drive fluorescent lamps are generally referred to as ballast circuits or “ballasts”. In practice, the term ballast is commonly used to refer to the entire fluorescent lamp drive circuit, and not just the current limiting portion.
Current flow through a fluorescent lamp is generally achieved by placing cathodes at either end of the lamp tube to inject electrons into a vapor within the lamp. These cathodes are structured as filaments that are coated with an emissive material used to enhance electron injection. The emission mix typically comprises a mixture of barium, strontium, and calcium oxides. A small electric current is passed through the filaments to heat them to a temperature that overcomes the binding potential of the emissive material allowing thermionic emission of electrons to take place. When an electric potential is applied across the lamp, electrons are liberated from the emissive material coating on each filament, causing a current to flow. While a lamp is in operation, and especially when a lamp is ignited, the emission mix is slowly sputtered off the filaments by bombardment with electrons and mercury ions. The rate of depletion of the emission mix varies from filament to filament. Thus as a lamp nears its end of life, the emission mix on one filament will deplete more quickly and exhibit lowered electron emissions, while the other filament will continue to support normal electron emissions. This can lead to a slight rectification of the alternating current flowing through the lamp. Continued operation of a lamp after the emission mix is depleted can lead to overheating resulting in cracking of the glass allowing hazardous mercury vapor to escape. It is therefore desirable to detect when a lamp is nearing its end of life (EOL) and turn it off before overheating can occur.
Temperature has a significant effect on the operation of fluorescent lamps. Wall temperature of a lamp affects the partial pressure of mercury vapor within the lamp, which in turn affects light output of the lamp. The wall temperature is generally a function of the ambient air surrounding the lamp, and other factors such as the room temperature or outside temperature where the lamp fixture is installed. Fluorescent lamps are typically designed to operate in ambient temperature environments that can from about 85 degrees Centigrade to 110 degrees Centigrade. At higher temperatures, such as for example above about 110 degrees Centigrade, fluorescent lamps are vulnerable to high currents that can damage the lamp and reduce its operational life. At lower temperatures, fluorescent lamps are generally harder to start and require a higher open circuit voltage from the ballast in order to reliably start at low temperatures. The minimum starting temperature of a fluorescent lamp can depend on both the rating of the lamp and of the ballast. Applying a starting voltage to a lamp that is higher than necessary can adversely affect lamp life. Thus, it would be advantageous to provide a lamp ballast configured to adjust the starting lamp voltage based in part on temperature.
During lamp operation, high temperatures can increase lamp current and undesirably reduce light output, lamp efficiency and lamp life. Accordingly it would be advantageous to provide a lamp ballast that can reduce high temperature effects and improve lamp life.
End-of life protection of a fluorescent lamp can also be improved by reducing or avoiding the high currents that can occur at higher lamp temperatures. Accordingly, it would be advantageous to reduce lamp current as lamp operating and ambient temperatures rise.
Accordingly, it would be desirable to provide a lamp ballast that addresses at least some of the problems identified above.